Stacked LCD color display

ABSTRACT

A color display is formed by stacking two or more birefringent elements that are tuned to provide different spectral characteristics, and operating each of the elements independently.

RELATED APPLICATION DATA

.[.The present invention.]. .Iadd.This application is a continuation ofReissue application No. 08/125,646, filed on Sep. 21, 1993, nowabandoned, which .Iaddend.is a continuation-in-part of application Ser.Nos. 07/402,134 filed Sept. 1, 1989, now U.S. Pat. No. 4,917,465,07/378,997 filed July 12, 1989, now U.S. Pat. No. 4,952,036, 07/363,099filed June 7, 1989, now U.S. Pat. No. 4,966,441, and 07/329,938 filedMar. 28, 1989, now abandoned. These applications are assigned to thepresent assignee and are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to systems for displaying color images,and more particularly relates to such systems wherein the image isformed by passing light through a plurality of birefringent opticalelements.

BACKGROUND AND SUMMARY OF THE INVENTION

Much effort has been made in recent years to develop low power colordisplays. Such efforts have generally employed LCD panels in one ofthree configurations. In the first configuration, a plurality ofdifferently colored LCD panels are stacked and illuminated with whitelight. As the light passes through the stacked layers, pixels in eachpanel act as controllable color filters, selectively coloring the lightexiting the display. U.S. Pat. No. 3,703,329 is representative of suchsystems and shows a stack of three panels, variously dyed toindividually produce the colors yellow, cyan and magenta Together thepanels cooperate, using subtractive color, to produce all eight primarycolors. A related system is shown in U.S. Pat. No. 4,416,514. In thissystem, differently dyed polarizers (yellow, magenta and cyan) areinterposed in a series of twisted nematic cells. By varying the voltageapplied to each cell, the twist angle of the liquid crystal moleculeschanges, imparting a variable rotation to the light exiting the cell.The colored polarizers cooperate with this controllably twisted light toselect desired colors.

While such stacked cell systems can provide a full color display, theytypically have certain drawbacks. One is parallax, inherent in anystacked optical system. Another is poor brightness, due to absorption oflight by the dye in dyed cell systems, and due to blockage of crosspolarized light by polarizers in systems that rely on polarizationrotation to differentiate colors.

The second approach uses only a single LCD panel, but uses it inconjunction with a mosaic color filter. The mosaic filter typically hasa plurality of red, green and blue filter elements, each aligned with apixel in the LCD panel. By controlling the transmissivity of pixels inthe LCD panel, the display can pass light through selected areas of thecolor mosaic filter.

While the color mosaic technique addresses certain shortcomings of thestacked panel approach, it introduces certain problems of its own. Oneis that brightness is limited because less than a third of the activepixel area transmits light for any given color.

Another shortcoming of the color mosaic approach is that pixel densitymust be increased by a factor of three to obtain the same resolution asthe stacked cell approach. That is, to provide a color display with ahorizontal resolution of 640 colored pixels, for example, the LCD panelsmust have 1920 pixels, 640 for each of the red, green and blue filterelements. This introduces fabrication problems that decrease yields andincrease panel costs. Further, the finite width of the gap betweenpixels must remain, even though the pitch has decreased, so the actualpixel "aperture ratio" can be decreased dramatically. (Some small formatthin film transistor (TFT) displays have a total open aperture area ofonly 45% of the total display surface due to row and column lines andtransistor area, etc.)

The third approach is birefringence color. In such systems, thebirefringent operating mode of certain material is exploited to producecolor, as opposed to reliance on colored dyes in guest-host type cellsor reliance on rotation of light through known twist angles in twistednematic cells.

Birefringent color systems typically take two forms: those relying onpassive birefringent layers to impart a birefringent effect to a liquidcrystal cell (as shown in U.S. Pat. No. 4,232,948), and those in whichthe liquid crystal material itself exhibits a birefringent effect(sometimes called "electrically controlled birefringence" or "tunablebirefringence"). In the latter instance, the degree of birefringence isa function of the voltage applied to the liquid crystal material. Byswitching the applied voltage to different values, different colors canbe produced. Color displays relying on this principle are shown in U.S.Pat. Nos. 3,785,721, 3,915,554 and 4,044,546.

During recent years, so called "supertwisted" or "highly twisted"nematic cells have become popular in many applications. Such cells aredescribed, inter alia, in U.S. Pat. Nos. 4,697,884 and 4,634,229, and inScheffer et al, "A New, Highly Multiplexable Liquid Crystal Display,"Appl. Phys. Lett. 45 (1), Nov. 15, 1984, pp. 1021-1023, and Kinugawa etal, "640×400 Pixel LCD Using Highly Twisted Birefringence Effect WithLow Tilt Angle," 1986 SID Digest, pp. 122-125. The '884 and '229 patentsare incorporated herein by reference.

Supertwisted nematic (STN) cells generally function in a birefringentmode. However, unlike earlier birefringent cells, STN cells exhibit abistable behavior wherein they switch rapidly from a deselect state to aselect state and back again as the excitation (RMS) voltage crosses aswitching threshold. The select and deselect voltage regions can be madequite close to one another, such as 1.20 volts and 1.28 volts,permitting the cells to be multiplexed at high rates. FIG. 1 shows thetransmission of a representative STN cell (with a particular polarizerorientation) as a function of applied voltage, illustrating thesteepness of the switching function. Note that this curve shows theoverall photopic "brightness" and does not reveal any coloration of theliquid crystal in the select and deselect states.

It is the multiplexibility of STN cells that makes them particularlydesirable. This multiplexibility is achieved without active elements(i.e. drive transistors on each pixel, etc.) and without exoticalignment and liquid crystal operating modes (i.e. ferroelectric,phase-change, hysteresis, etc.). Thus, STN provides an inexpensivedirect-multiplexed display type requiring only M+N drivers to operate adisplay comprised of M×N pixels.

There is an inverse-squared relationship between the number of displaylines to be "addressed" and the difference between display "on" and"off" driving voltages (RMS average). As the number of display linesincreases, the difference in driving voltage must decrease. Toillustrate, a multiplex rate of 100:1 can be achieved with approximatelya 10% difference in driving voltages, and a MUX rate of 240:1 can beachieved with approximately a 6% difference in driving voltages.Theoretically, arbitrarily high MUX rates can be achieved if the drivingvoltage difference is made small enough.

The main drawback to STN is the optical operating mode--birefringence.That is, the only way to distinguish pixels driven by the "on" voltagefrom those driven by the "off" voltage is the difference inbirefringence between the two pixels. (As noted, for high informationcontent displays, the difference in driving voltages is minute anddecreases rapidly with an increase in the number of display lines thatmust be driven.) To distinguish the difference in pixel birefringence,polarizers are used--one to polarize the entering light to a knownpolarization, and one to select only one polarization of exiting lightfor examination. Depending on the state of the pixels, the lightoriented to pass through the exit polarizer will be one of two colors.For best contrast, the polarizers are usually arranged so that these twocolors are yellow and blue. (Actually, only one color can be selected byorientation of the polarizers--and this color can only be selected froma relatively small spectrum of colors. There is very little designfreedom in varying the color in the second state--it is essentially afunction of the first color.)

FIG. 2 shows the transmission characteristics of a representativeyellow/blue mode STN cell (with associated polarizers) when the cell isin its select and deselect states. As can be seen, when the cell is"selected" (by applying an excitation voltage of 1.56 volts), thetransmission spectrum has a maximum at 400 nanometers, a minimum at 600nanometers, and an intermediate value at 500 nanometers. When the cellis "deselected" (by reducing the excitation voltage to 1.41 volts), thetransmission spectrum includes a null at 400 nanometers, a maximum at500 nanometers, and an intermediate value at 600 nanometers. Lightexiting the cell/polarizer combination in the select state is thusprincipally blue, and light exiting in the deselect state is green plusyellow plus red, which appears as yellow to the human observer.

Unlike TN cells and cells operating in other modes, a birefringent STNcell cannot be operated in a black/white mode. The reason is that blackrequires all wavelengths of light to be linearly cross-polarized withthe exit polarizer to effect complete light blockage, and true whiterequires all wavelengths of light to be linearly polarized parallel withthe exit polarizer to effect complete light passage. The birefringentoperating mode, by definition, prevents such results since differentwavelengths of light are polarized differently during passage throughthe material. Thus, STN cells are unavoidably colored. However, thisdrawback has been tolerated in order to achieve the highmultiplexibility that STN provides.

In order to eliminate the birefringence color, some manufacturers haveincorporated various compensation layers in display assemblies. One suchcompensation layer is a second birefringent cell of opposite twist thanthe first to counteract the wavelength dependence in the cell'sbehavior. Another type of compensation layer, sometimes used inconjunction with the above-mentioned blue/yellow mode STN LCDs, is apolarizer that has been dyed to pass cross-polarized light in the blueand red portions of the spectrum in order to make the yellow state ofthe LCD "whiter." This still yields a blue/white LCD, instead of thedesired black/white. However, this color limitation is usually acceptedin order to achieve the high multiplex ratio.

While the birefringence of STN cells unavoidably produces colors, thecolors so produced are generally considered too limited in range and tooinferior in quality to be suitable for use in color displays. Farpreferred are the rich colors that can be achieved with guest-hostcells, or TN cells with dyed filters.

In accordance with the present invention, a color display system isformed by operating a plurality of birefringent cells in cooperationwith one or more colored polarizers, thereby complementing andcorrecting the birefringence colors and yielding a brighter display.

In a preferred embodiment, a plurality of STN birefringent panels aretuned to different subtractive primary colors (i.e. yellow, cyan andmagenta) and stacked. Interposed between the panels, and sandwichedabout the stack, are polarizers, at least one of which is colored. Insome embodiments, this assembly is illuminated by a collimated lightsource and the resulting image is focused onto a projection screen forviewing. In other embodiments, optics are provided to permit direct wideangle viewing of the display without parallax effects.

The foregoing and additional features and advantages of the presentinvention will be more readily apparent from the following detaileddescription, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the transmission characteristics of a representative STNcell as a function of applied voltage.

FIG. 2 shows the transmission spectrum of a representative STN cell whenoperated in its select state (with an excitation voltage of 1.56 volts)and in its deselect state (with an excitation voltage of 1.41 volts).

FIG. 3 is a schematic diagram of a display subassembly according to oneembodiment of the present invention.

FIGS. 4-6 are spectral photometer plots showing ideal light transmissioncharacteristics for three liquid crystal panels used in the displaysubassembly of FIG. 3 when in their selected and deselected states.

FIGS. 7-9 are spectral photometer plots showing the actual lighttransmission characteristics of three Kyocera liquid crystal panels usedin the display subassembly of FIG. 3 when in their selected anddeselected states.

FIG. 10 is a chromaticity diagram illustrating the performance of theKyocera panels when in their selected and deselected states.

FIG. 11 is a diagram showing the eight basic colors achieved byoperating yellow, cyan and magenta panels in their various combinations.

FIG. 12 details the construction of a display assembly incorporatingthree panels according to the present invention.

FIG. 13 shows a first projection system according to the presentinvention.

FIG. 14 is a perspective view of an integrated assembly including adisplay assembly and associated optics to facilitate use with anoverhead projector.

FIG. 15 shows a second projection system according to the presentinvention.

FIG. 16 shows a self contained color display using a display subassemblyaccording to the present invention with associated projection optics.

FIG. 17 shows a first direct view display system according to thepresent invention.

FIG. 18 shows the spectral distribution of a backlight that may be usedwith the display system of FIG. 17.

FIG. 19 shows a second direct view display system according to thepresent invention.

FIG. 20 shows a third direct view display system according to thepresent invention.

FIGS. 21 and 22 show a fourth direct view display system according tothe present invention.

FIG. 23 shows a portable computer employing a direct view displayaccording to the present invention.

FIG. 24 shows a laptop computer employing a direct view displayaccording to the present invention.

FIG. 25 is a view of a portable computer including a direct view displayaccording to one embodiment of the present invention.

FIG. 26 is a perspective view of the portable computer of FIG. 25.

FIG. 27 is a view of a portable computer including a direct view displayaccording to another embodiment of the present invention.

FIG. 28 is a perspective view of the portable computer of FIG. 27.

FIG. 29 is a view of a portable computer including a direct view displayaccording to yet another embodiment of the present invention.

FIG. 30 is a perspective view of the portable computer of FIG. 29.

FIG. 31 shows a display stand that permits a display subassembly to bebacklit for direct viewing.

FIG. 32 shows a display system employing two light sources and twooptical paths according to the present invention.

FIG. 33 shows a display system employing one light source and twooptical paths according to the present invention.

FIG. 34 shows a computer with a roll-up screen according to the presentinvention.

FIG. 35 shows a display subassembly using a thin film transistor (TFT)LCD panel in conjunction with an STN panel.

FIG. 36 shows a display subassembly using a TFT panel, an STN panel, anda color shutter.

FIG. 37 shows a display subassembly using two panels.

FIG. 38 illustrates a possible color gamut produced by one of the panelsof FIG. 37.

FIG. 39 illustrates the gamut of FIG. 38 after being analyzed with ablue polarizer.

FIG. 40 illustrates the spectral characteristics of two possiblepolarizers used in the display subassembly of FIG. 37.

DETAILED DESCRIPTION

To provide an enabling disclosure without unduly lengthening thisspecification, applicants incorporate by reference the disclosures ofU.S. Pat. Nos. 4,549,174, 4,652,101, 4,709,990, 4,763,993 and 4,832,461which teach certain concepts useful in the construction of a deviceaccording to the present invention.

Before proceeding, it may be helpful to first review certain principlesof color optics. The primary light colors are generally considered to bered, green and blue. White light is composed of all three primaries.White light with red filtered therefrom (i.e. removed) is termed cyan;white light with green filtered therefrom is termed magenta; and whitelight with blue filtered therefrom is termed yellow. These lattercolors, cyan, magenta and yellow, are sometimes termed subtractiveprimary colors, since they denote the absence of one of the primarycolors.

Filters selectively attenuate (or "absorb") light of certain colors andpass light of other colors relatively unattenuated. A red filter, forexample, attenuates blue and green light and lets red light pass.Similarly, a blue filter attenuates red and green light and lets bluelight pass. Finally, a green filter attenuates red and blue light andlets green light pass. Filters of the primary colors thus subtract twoprimary colors and let the third pass.

Filters of the subtractive primary colors subtract one primary color andlet the two others pass. For example, a cyan filter attenuates red lightand lets blue and green light pass. Similarly, a magenta filterattenuates green light and lets blue and red light pass. Finally, ayellow filter attenuates blue light and lets green and red light pass.

These properties are summarized in Table I.

                  TABLE I                                                         ______________________________________                                        Filter        Absorbs       Passes                                            ______________________________________                                        Red           Green, Blue   Red                                               Green          Red, Blue     Green                                            Blue           Red, Green    Blue                                             Yellow         Blue           Green, Red                                      Cyan           Red            Blue, Green                                     Magenta       Green           Blue, Red                                       ______________________________________                                    

Again, although somewhat counter-intuitive, it should be remembered thata blue filter does not absorb blue light. It passes blue light andblocks light of other colors.

It should further be noted that the human eye is more sensitive tocertain wavelengths of light than to others. The eye's normal daytimeresponse (termed "photopic" response) typically peaks at about 554nanometers and diminishes to near negligible values around 400 and 700nanometers.

For convenience of discussion, the optical spectrum is generallysegregated into the red, green and blue portions by dividing lines at500 and 600 nanometers. (For physiological reasons, a precise dividingline cannot be defined.) Using these boundaries, the human eye perceives55% of the energy in white light from the green portion of the spectrum(500 to 600 nm.), 30% from the red portion (above 600 nm.), and only 15%from the blue portion (below 500 nm.). Perfect green, red and bluefilters thus transmit 55%, 30% and 15% of white light, respectively(photopically). Since yellow, cyan and magenta are combinations of thesecolors, it can be seen that perfect yellow, cyan and magenta filterstransmit 85%, 70% and 45% of white light, respectively.

The LCD panels used in the illustrated embodiments are supertwistednematic LCD panels that are controllably colored by exploitation of thebirefringence effect. As mentioned in the Background discussion,birefringence is an optical phenomenon in which light oriented along oneaxis of the material propagates at a different speed than light orientedalong another axis. This asymmetry results in different wavelengths oflight having different polarizations when they exit the material.Polarizers can be used to analyze the elliptically polarized lightexiting the panel to select colors. Prior art uses of birefringence tocontrol color in LCD panels are discussed in U.S. Pat. Nos. 3,876,287,4,097,128, 4,127,322, 4,394,069, 4,759,612 and 4,786,146, thedisclosures of which are incorporated by reference.

Display Subassembly

Turning now to FIG. 3, there is shown a basic display subassembly 10according to one embodiment of the present invention. The illustratedsubassembly includes four LCD panels 12, 14, 16, 18 sandwichedalternately between five polarizers 20, 22, 24, 26 and 28. An optionalretardation film layer 30 is also shown.

In the illustrated subassembly 10, the birefringent properties of thepanels 12-18 are "tuned" (by choosing the thickness (d) of the liquidcrystal layer and its optical refractive index anistropy (Δn)) to yielda desired coloration. For example, the birefringent properties of thefirst panel 12 are tuned so that incoming green light (which has beenpolarized by the entrance polarizer 20) propagates through the liquidcrystal material in such a manner that the orientation of its principalaxis upon leaving the cell is orthogonal to the exiting polarizer 22when the panel 12 is in its deselected (i.e. deenergized) state. Thepanel 12 and polarizers 20 and 22 thus act as a magenta filter when thepanel is deselected. The tuning of panel 12, and the orientations of theassociated polarizers, are also selected so that, when the panel is inits selected (i.e. energized) state, green light is passed, togetherwith red and blue light, to yield a substantially "white" color. (Forexpository convenience, panel 12 is sometimes called the "magenta" paneland is said to controllably absorb green light. It will be recognized,however, that this and the other panels must be operated in conjunctionwith associated front and back polarizers to achieve the desiredcoloring effect.)

The illustrated second panel 14 is similarly tuned to operate as ayellow filter (i.e. absorbing blue) when in its deselected state and topass all wavelengths of light (i.e. white light) when in its selectedstate. It is sometimes termed the "yellow" panel. The illustrated fourthpanel 18 is similarly tuned to operate as a cyan filter.

The illustrated third panel 16 is an optional "black" panel that may beincluded to increase contrast. Its construction may take any of a numberof forms, as discussed below.

As mentioned earlier, it is the thickness (d) of the liquid crystallayer and its optical refractive index anistropy (Δn) that principallydetermine each panel's spectral response for a given twist angle ψ.While the panel's response is determined by a complex formula, theresponse can be roughly approximated as dependent on the ratio Δnd/ψ. Inthe illustrated embodiment, these ratios have the values shown in TableII:

                  TABLE II                                                        ______________________________________                                        Panel      Δnd/Ψ  Ψ                                             ______________________________________                                        Magenta    0.19             4.19 (rad.)                                       Yellow      0.23              3.84                                            Cyan        0.25              4.19                                            ______________________________________                                    

It will be recognized that the Δnd/ψ ratios referenced in Table II canbe achieved with any number of cell thicknesses. The choice of cellthickness is a tradeoff between several factors, including the panel'sresponse time and uniformity. The response time of the panel generallyincreases with the panel thickness. Consequently, to achieve a fastresponse time, it is desirable to use a thin panel. However, as the cellthickness decreases, small fabrication errors, such as a 1μ change incell thickness over the width of a panel, yields a relatively largevariation in panel color behavior and switching threshold voltage. Toinsure color uniformity, it is desirable to use a thick panel sofabrication errors are kept to a small percentage of the total liquidcrystal thickness. As a compromise between these considerations, a cellthickness of 6 to 12μ may be used.

Spectral photometer plots showing the light transmission qualities ofideal panels 12, 14 and 18 (again, considered in conjunction with theirassociated polarizers) are provided in FIGS. 4, 5 and 6, respectively.Panels suitable for use as panels 12, 14 and 18 are available fromKyocera of Hayato, Japan as part numbers KC-6448ASTP-SC-M,KC-6448ASTP-SC-Y and KC-6448ASTP-SC-C, respectively, or may befabricated using known techniques. Spectral photometer plots showing theactual behavior of the Kyocera panels are provided in FIGS. 7-9. Theplot for the magenta panel in FIG. 7 was made with a red entrancepolarizer. The plot for the cyan panel in FIG. 9 was made with a blueexit polarizer. (As can be seen from these curves, neither the passageof light of the desired color nor the attenuation of light of undesiredcolors is perfect, but the resulting effect is more than adequate toprovide saturated colors throughout the human visual area.) Achromaticity diagram illustrating performance of the Kyocera panels intheir selected and deselected states is provided in FIG. 10.

Each of panels 12-18 comprises a plurality of pixels that can beindividually energized to change the spectral distribution of the lightthat is permitted to pass therethrough. By selecting correspondingpixels in the three colored panels, light of any color can betransmitted through the display subassembly 10. To transmit a pixel ofgreen light, for example, a pixel in the yellow panel 14 is deselectedto absorb blue light and the correspondingly positioned pixel in thecyan panel 18 is deselected to absorb red light. By superimposing thespectral transmission curves of these two pixels, it will be recognizedthat the remaining, transmitted light has a peak in the region of thespectrum the eye perceives as green. (The magenta panel 12 is leftselected (i.e. white transmitting) in this example and thus has norelevant filtering effect.)

The color blue can be similarly achieved by deselecting correspondingpixels in the cyan and magenta panels, and red can be achieved bydeselecting corresponding pixels in the yellow and magenta panels. If itis desired to absorb all light and thus produce a black pixel on theimage plane, pixels in all three panels are deselected. FIG. 11 showsthe eight basic colors achieved by operating a yellow/cyan/magentaseries of panels in their various combinations.

Polarizers are needed to analyze the light passing through the liquidcrystal panels in order to achieve perceptible contrast. In prior artsystems, the polarizers are typically neutral (i.e., dyed black byiodine). In the present invention, colored polarizers (which are"leaky") can be used in certain positions to pass more light, improvingthe brightness and allowing color balance improvements.

The first panel 12 is illustrated as being "magenta." Light entering itis polarized by the first polarizer 20. Normally, all colors of lightorthogonal to the axis of polarizer 20 would be absorbed by the blackdye of a conventional, neutral polarizer, resulting in an immediate lossof 50% (theoretical) of the light. (In actual practice, the loss of aneutral polarizer is about 55-58%.) This loss can be cut dramatically ifthe first polarizer is dyed magenta. Such a polarizer still passes thewhite light parallel to the polarizer's axis, but additionally passesblue and red light orthogonal to its axis. This additional blue and redlight is permitted to pass further into the display subassembly andultimately contributes to the overall brightness of the resultingdisplay, instead of being absorbed by the first polarizer as is normallythe case. The losses normally associated with this first polarizer arethus cut by about two thirds. Display brightness improvescommensurately.

(In an alternative embodiment, the entrance polarizer 20 may be dyedred. While theoretically not as advantageous as a magenta polarizer, ared polarizer is easier to realize and still offers a substantialimprovement in brightness, passing about 59% of the incident light, asopposed to 45% or less for a neutral polarizer.)

The same benefit can be achieved at the exiting end of the sandwicheddisplay subassembly 10. The last panel 18 in the subassembly isillustrated as being cyan. By dying the polarizer 28 adjacent theretocyan, the blue and green light that would normally be absorbed therebyis allowed to leak through and pass out of the display subassembly,again improving display brightness.

(Again, the exit polarizer 28 may be dyed blue instead of cyan. A bluepolarizer passes about 56% of the incident light, still yielding asignificant improvement in brightness over a neutral polarizer.)

Conventional neutral polarizers can be used at the positions (22, 24,26) intermediate the liquid crystal panels and a significant improvementin display brightness is still achieved by virtue of the two coloredpolarizers described above. The use of neutral intermediate polarizersalso assures that there is no birefringence interaction between panels(i.e. the deselected or selected nature of the Δnd of the center panelmakes no difference to the passage of light and total birefringence ofthe adjacent panels).

In other embodiments, the polarizers at the intermediate positions inthe subassembly may be colored. Care must be taken, however, not tointerfere with the color-selective properties of the birefringentpanels. For example, if a yellow colored polarizer is interposed betweenthe magenta and yellow panels 12, 14, it will interfere with thecolor-selective properties of the magenta panel. As noted, the magentapanel itself does not absorb the undesired green light. Instead, itsbirefringence is tuned so that light propagating through the panel exitswith the axis of its principal green component oriented orthogonally tothe polarizer 22, causing it to be blocked. If this polarizer 22 iscolored yellow, it will leak green and red light, including the greenlight that is meant to be blocked. Consequently, use of a yellowpolarizer between the magenta and yellow panels defeats the carefultuning of the first panel's birefringence.

An equally poor color choice for the first intermediate polarizer 22 ismagenta. A magenta polarizer would permit blue and red light to enterthe yellow panel 14 at an unexpected orientation. The yellow panel wastuned so that blue light entering at a known polarization wouldpropagate and exit with a principal polarization that would be blockedby the exiting polarizer 24. If the blue light enters the yellow panel14 at an unexpected orientation, it will exit at an unexpectedorientation and will not be blocked by the exiting polarizer.Consequently, use of a magenta colored polarizer 22 defeats the carefultuning of the yellow panel's birefringence.

Polarizer 22 should be colored, if at all, a color that both of theadjoining panels are intended to pass. In this case, since the magentapanel is intended to pass blue and red, and the yellow panel 14 isintended to pass green and red, the polarizer 22 should be colored thecommon color: red.

If the black panel 16 is omitted (together with associated retardationfilm 30 and polarizer 26), similar logic would dictate that thepolarizer 24 between the remaining yellow and cyan panels should becolored, if anything, green.

In embodiments including a black/white panel, such as panel 16 in FIG.3, the polarizers positioned adjacent thereto should be neutral (i.e.not colored) since any polarizer coloring would permit the black panelto leak light--an undesired effect.

To optimize display brightness, the dyed polarizers should exhibit ahigh degree of transmissivity to cross-polarized light in their "leaky"portion of the spectrum. In the illustrated embodiment, the polarizerseach comprise a dyed 5 mil sheet of stretched polyvinyl alcohol. TableIII specifies suitable dichroic dyes, which are available under variousbrand names from Crompton & Knowles, Atlantic, Ciba-Geigy and a varietyof other dye suppliers.

                  TABLE III                                                       ______________________________________                                        POLARIZER          DYE                                                        ______________________________________                                        Magenta            Direct Red #81                                             Yellow               Direct Yellow #18                                        Cyan                 Direct Blue #1                                           ______________________________________                                    

The foregoing discussion has described only one of many possiblesequences of polarizers and panels. Others can be devised. For example,while the first polarizer 20 in the above example has been described asbeing magenta in order to achieve an improvement in brightness, analternative embodiment with the same sequence of LCDs can here use ablue or red polarizer instead. A blue or red polarizer still providessome improvement in brightness since it leaks light that would beabsorbed by at black polarizer. Of course, a black polarizer can also beused if desired. The basic LCD sequence itself can also be varied withcorresponding changes in the associated polarizers. The basic sequencesare set forth in Table IV:

                  TABLE IV                                                        ______________________________________                                        POL1   LCD1     POL2   LCD2   POL3 LCD3   POL4                                ______________________________________                                        Y/G/R/K                                                                              Y        G/K    C      B/K  M      M/R/B/K                             M/R/B/K                                                                              M        R/K    Y      G/K  C      C/G/B/K                             Y/G/R/K                                                                              Y        R/K    M      B/K  C      C/G/B/K                             ______________________________________                                    

where Y is yellow, K is black, G is green, C is cyan, B is blue, M ismagenta and R is red.

FIG. 12 illustrates in greater detail a display subassembly using justthe magenta, yellow and cyan panels. The polarizers are magenta, black,black and cyan, respectively. Included in FIG. 12 are details of therelative alignment of the component panels and polarizers in animplementation using the Kyocera panels. The alignment angles aretypically specified by the manufacturer and depend, inter alia, on therubbing angles of the front and rear panel plates, the twist of the LCDmolecules, and on various boundary layer phenomena associated with theliquid crystal material.

As noted, such a three panel subassembly can produce the color "black"(the absence of light) by deselecting each panel. Since the lightpassing through the subassembly is progressively stripped of its green,blue and red components, theoretically no light exits the subassembly.As a practical matter, however, the imperfect responses of the threepanels permit some light of various colors to leak through at anattenuated level. The net result is a dark brown or grey color. Whilesuch an arrangement yields a contrast ratio of approximately 10:1--morethan adequate for many applications--some applications require contrastratios on the order of 100:1. To achieve such ratios, a fourth panel,such as the "black" panel 16 illustrated in FIG. 3, may be included inthe subassembly. The characteristics of the black panel may be optimizedfor the intended application.

In one application, namely digital computer graphics using the RGBIstandard, an "intensity" signal is used to differentiate each of theeight basic colors used in RGB systems into two colors, yielding a totalof 16 colors. In such application, the black cell is optimized formaximum transmission when in the selected state. The contrast providedby the cell is of lesser importance. That is, a contrast range of 2:1;or even 1.5:1, will suffice to distinguish the 16 colors of the RGBIsystem.

In contrast, "full color" systems (i.e. television or high quality colorcomputer graphics) require high contrast. To achieve the 256 or morecolors that such systems require, an overall contrast ratio of 100:1 maybe needed. Since the basic magenta/yellow/cyan (M/Y/C) subassemblydelivers only a 10:1 contrast ratio, the black panel must provide a 10:1ratio on its own. Thus, it must be optimized for blackest black. Bycascading the two systems (M/Y/C and black), the contrast figures aremultiplied, producing 100:1 overall white to black contrast, andallowing excellent grey shading and range of color. Of course, highlysaturated primary colors still require M/Y/C contrast, but the blackpanel provides greater depth in the shadows and details in thehighlights.

In the illustrated embodiment, the black panel 16 is a supertwistednematic cell operated in conjunction with a retardation film 30 thattunes the cell for maximum contrast. In other embodiments, a doublesupertwisted nematic cell or even a twisted nematic cell may be used.

In actual practice, the "black" cell need not be black. A birefringentcell tuned to the blue end of the spectrum, for example, may be usedsince the human eye is relatively insensitive to blue light, yielding arelatively high photopic contrast ratio.

One advantage of the display subassembly of the present invention is theflexibility it affords in possible panel/polarizer sequences. If onesequence seems unworkable, a design can be optimized about another one.For example, if it is found that a good quality magenta polarizer cannotbe obtained, then a design that does not require a magenta polarizer canbe adopted.

It will be recognized that a display subassembly 10 according to thepresent invention can be used in a variety of applications, such ascolor projection systems and in direct view displays. A variety of suchapplications are detailed below.

Projection Systems

In a first projection system embodiment 32 of the invention, shown inFIG. 13, a display subassembly 10 is positioned on the transparentprojection surface 34 of a conventional overhead projector 36. Suchprojectors typically include an illumination bulb 38 and a Fresnel lens40 under the projection surface to produce light beams that pass througha transparency and converge onto a projection lens assembly 42. (Due tothe short focal length and high power required of lens 40, it is oftenformed by cementing two or more lower powered Fresnel lenses together.)

When display subassembly 10 is used in such an embodiment, it isdesirable to provide a Fresnel lens 44 to collimate the converging lightfrom the projection surface 34 prior to illumination of the displaysubassembly. The light exiting the subassembly is then focused by a lens46 (which is also desirably in Fresnel form) onto the projection lensassembly 42. (Lens 46 here serves the same purpose as the Fresnel lens40 provided under the projection surface of the projector in theprojector's normal operation, namely to focus light towards theprojection lens assembly 42.) An integrated assembly 47 including boththe display subassembly. 10 and the Fresnel lenses 44, 46 is shown inFIG. 14.

In a second projection system embodiment 48 of the invention, a portionof which is shown in FIG. 15, the collimating and focusing Fresnellenses 44, 46 used in the FIG. 13 embodiment are omitted. Instead, thepanels comprising the display subassembly are fabricated with differentpixel spacings. The spacings on the various panels are selected so thatcorresponding pixels in the various panels are aligned with theconverging light exiting the projection surface of the projector. Bythis arrangement, no accessory optics are required. Parallax effects areavoided since the internal optics of the display subassembly aredesigned to cooperate with the focused light used by the projector.

Projection technology may also be used to provide a self containeddisplay in which an image is projected onto the rear of a viewingscreen. A color monitor for a computer may be realized in this fashion.One such arrangement 50 is shown in FIG. 16. In this embodiment, a fieldlens 52 is used to collimate the light from bulb 54 prior to its passagethrough the display subassembly 10. The resulting image is projected bya second lens 56 onto a translucent medium 58 which can then be viewedfrom the opposite side by a user.

Direct View Systems

A display subassembly 10 according to the present invention can also beincorporated into a number of direct view display systems, such as colorgraphics displays for portable or laptop computers.

In direct view displays, it is usually desirable to backlight thedisplay with substantially collimated light. On the viewing side of thedisplay, it is desirable to provide exit optics that permit a wideviewing angle without parallax effects.

In a first direct view embodiment 60, shown in FIG. 17, the displaysubassembly 10 is backlit from a diffused light source, such as afluorescent light panel 62. In such embodiment, entrance and exit opticelements 64, 66 generally collimate the diffuse light prior to entranceinto the display subassembly and scatter the approximately-collimatedlight exiting the display. Each of optic elements 64, 66 may comprise aplate having formed thereon a plurality of microlenses 68, one alignedto each pixel. Light incident on one of the microlenses on element 64 isdirected substantially normal to the plane of the subassembly and thuspasses through the pixels of the component panels in the properalignment, regardless of its initial orientation. Collimated lightexiting the subassembly 10 is dispersed by the microlenses on the exitoptic element 66, thereby permitting the color image to be viewed from awide range of angles without parallax effects. The interstitial areas 69between the lenses on exit optic 66 may be colored black to minimizestray light and to improve perceived contrast.

In other versions of the FIG. 17 embodiment 60, the arrays ofmicrolenses can be replaced by arrays of fiber optic collimatorfaceplates or lenticular lenses.

FIG. 18 shows the spectral distribution of a representative fluorescentbacklight 62 that may be employed in the embodiment of FIG. 17. As ischaracteristic of florescent lighting, the spectrum has characteristicpeaks corresponding to certain chemical components used in the light.These peaks (and the nulls) can be tailored to specific applications bychanging the chemistry of the lamp.

In a second direct view embodiment 70, shown in FIG. 19, the backlitillumination can be collimated by a novel arrangement employing aparabolic mirror 72 (desirably in Fresnel form). In this embodiment, apair of linear light sources, such as fluorescent bulbs 74, illuminate agenerally flat mirrored surface 76 that has facets arranged to provideone axis of collimation. The angles of the facets vary with placement onthe surface to simulate a sectioned parabolic reflector. Light reflectedfrom this mirrored surface is substantially collimated. However, toremove any stray off-axis light, a microvenetian blind material 78, suchas Light Control Film marketed by 3M Corp, is desirably placed betweenthe mirror and the display subassembly. This material is a thin plasticfilm containing closely spaced black microlouvers to absorb lightmisaligned with respect to the louvers. Substantial collimation of theilluminating light is thus achieved.

In the FIG. 19 embodiment 70, a translucent light dispersing material80, such as a ground glass plate or a commercially available diffusionmaterial (i.e. Rolux film manufactured by Rosco of Port Chester N.Y.) ismounted adjacent the exit side of the display subassembly 10 to displaythe resulting color image.

FIG. 20 shows a third direct view embodiment 82 of the invention. Inthis system, the display subassembly 10 is illuminated by atungsten-halogen lamp 84 that operates in conjunction with a curvedreflector 86. The reflector is computer designed (using well knownoptical modeling programs or ray tracing techniques) to provide equalenergy illumination to all regions of the display subassembly. Acorrector plate 88, mounted adjacent the display subassembly, provides anormalization of illumination angle, i.e. perpendicular to the assembly.

The lamp 84 in the FIG. 20 embodiment is desirably part of a removablemodule that also includes a shield 90 for preventing direct illuminationof the display subassembly by the lamp. Again, a diffuser material 92 ismounted adjacent the exit side of the display subassembly to permitdirect, wide angle viewing.

A fourth direct view embodiment 94 of the invention is shown in FIGS. 21and 22 and includes fiber optic backlighting of the display subassembly.In the illustrated system, a tungsten-halogen lamp 96 is again used, butthis time is optically coupled to a bundle of optical fibers 98. Eachfiber terminates at a microlens 100 on a plate 102 of such microlenses.These microlenses can be arrayed in a rectangular pattern on the plate102, or can be arranged in a hexagonal pattern for higher density. Ineither event, the microlenses are matched to the dispersion patterns ofthe fiber so that light exiting the fibers is substantially collimatedby the lenses. Again, a diffuser optic 104 is desirably positionedadjacent the exit side of the display subassembly.

In the FIG. 21, 22 embodiment, it is desirable that tolerance beprovided for non-uniformities, and maximum use be made of all light,including paraxial rays. Fortunately, with the intimately contactingdiffuser 104 on the top surface of the display subassembly, there is areasonable "blur" tolerance. Some stray light can even be beneficial to"intialias" the jagged square pixels.

The above-described direct view displays may each be advantageouslyincorporated into a portable or laptop computer. "Portables" aregenerally considered to be computers that are sized for readyportability, but still require use of 120 volts AC from a wall outletThey often take a suitcase-like form. "Laptops," on the other hand,generally rely on an internal rechargeable battery and often take a"clamshell" form.

FIG. 23 shows a portable computer 106 including a direct view display108 according to the present invention. To operate the computer, thecase 112 is opened and the display is positioned for viewing. (In someportables, the display is coupled to the computer by a coiled cable andcan be positioned where desired.) When the computer is no longer needed,the display is packed into the case, secure against abuse.

FIG. 24 shows a laptop computer 114 including a direct view display 108according to the present invention. As can be seen, the display iscoupled to the remainder of the computer by a hinge arrangement 115. Thelaptop's internal rechargeable battery 105 powers both the computer andthe display.

Again, to operate the computer 114, the hinged display 108 is lifted,exposing it for viewing. When the computer is no longer needed, thehinged display is secured in its collapsed position, protecting it fromabuse.

FIGS. 25-30 illustrate a variety of other portable computer designs thatare adapted for use with a display subassembly according to the presentinvention. In FIGS. 25 and 26, a computer 200 includes a displaysubassembly 10 mounted by a hinge 202 to the front top edge of acomputer case 204. When in use, the display subassembly 10 isilluminated by light reflected off a mirrored surface 206 from a lamp208. The lamp 208 is a point source (i.e. it has a relatively smallphysical extent, such as a small filament) and is fixedly attached tothe body of the computer case 204. The diverging light from this pointsource is collimated by a flat lens (not particularly shown in thefigures) mounted adjacent the display subassembly.

To fold for storage, the display subassembly 10 on computer 200 pivotsrearwardly into the body of the computer case, and the panel 210 towhich the mirrored surface is attached folds down over the display,protecting it from abuse. The computer keyboard 212 slides into a recess214 in the front portion of the computer case and a door 216 closes tosecure the keyboard in place.

FIGS. 27 and 28 show a portable computer 218 in which the displaysubassembly 10 is illuminated by light reflected from a mirror 220 thatslides out the back of the computer case 222. Again, the illumination isprovided by a point source, such as a tungsten-halogen bulb 224 that ismounted to the computer case 222 rather than to display subassemblyitself.

In operation, the display subassembly is positioned in a substantiallyvertical orientation on a hinge 226 at the rear top portion of the case.To collapse for storage, the display subassembly folds forwardly andlatches in place over the keyboard 228. The mirror 220 is slid towardsthe case and locks with the mirrored surface adjacent the case's backside. (In another embodiment, the mirrored surface is small enough to bepositioned entirely within the computer case. In still anotherembodiment, the illustrated mirror is hinged at point 230, permitting itto be folded flat and slid entirely within the computer case.)

Again, a flat correction lens is desirably mounted on the rear of thedisplay subassembly to collimate the light reflected from the mirror220.

FIGS. 29 and 30 show a portable computer 232 in which the displaysubassembly is directly illuminated from a point source 234, without anintervening mirror. In this embodiment, the display subassembly 10 isagain attached by a hinge 236, this one in a cavity 240 in the frontportion of the computer case 238. In use, the display subassembly 10 ispositioned substantially vertically and is illuminated by the pointsource 234. To fold for storage, the display subassembly 10 foldsrearwardly into the cavity and is held secure by the keyboard 242, whichis inverted and latched into place to serve as a top cover.

FIG. 34 shows yet another computer 250 according to the presentinvention. In computer 250, a point light source 252 is disposed withina case 254 and illuminates a display subassembly 256, which fills anaperture formed in the case. An image formed by projection of lightthrough this display subassembly is projected on a screen 258 positionedat the back of the computer case 254. In the illustrated embodiment, theprojection screen is flexible and is rolled for storage about aspring-loaded roller 260 disposed at the bottom rear portion of thecomputer case. To erect the screen for viewing, a screen support 262 ispivoted upwardly from its collapsed storage position to an uprightposition at the back of the computer. The unrolled screen can then befastened to the screen support by one or more clips, or like means (notshown).

It will be recognized that in all of the foregoing embodiments, thepixel pitches on the various panels may be made different (as shown inFIG. 15) to align the pixels with the orientation of the incoming light.By such a construction, it is not necessary to collimate or otherwiseprocess the light prior to illumination of the stacked subassembly. (Itwill further be recognized that the invention can be practiced by simplyilluminating a stack of uniformly pitched panels with uncollimated 55light, although parallax effects may cause improper pixel registration,blur and false color edges.)

FIG. 31 shows a final embodiment 116 illustrating use of a displaysubassembly 10 according to the present invention in a direct viewdisplay. In this embodiment, the display subassembly is removablypositioned on an illumination stand 118 for direct viewing. Theillumination stand 118 has a light-transmitting surface 120 againstwhich the display subassembly can rest, and an internal light source 122for directing illumination therethrough. A small shelf 124 on which thedisplay can be positioned is provided on the exterior of the stand.

The stand 118 is desirably collapsible to permit ready portability. Thiscan be achieved with a hinge and bellows arrangement 126. Small size canbe maintained by using folded optics that include mirroring on theinside back wall 128 of the stand.

The stand may be provided with optics that emulate the optics of aconventional overhead projector. That is, these optics may focus lightincident on the display 10 so that it converges on a point a shortdistance away. In the illustrated embodiment, these optics may comprisea Fresnel plate lens 130. In such case, the Fresnel entrance optic 44used in the FIG. 13 projection system embodiment may be used tocollimate the focused light prior to its illumination of the displaysubassembly.

In this embodiment, the exit optic 132 is again a simple translucentdispersion medium to permit wide angle viewing of the collimated image.

It will be recognized that the viewing stand 118 advantageously permitsan LCD display to be used either as a projection device for largeaudiences (i.e. as an "electronic transparency"), or as a single-usercomputer screen.

Systems with Split Optic Paths

While the foregoing discussions have been directed to displaysubassemblies comprised of single stacks of panels, additionaladvantageous results can sometimes be achieved by splitting the stackinto two or more sub-stacks and illuminating each separately. One sucharrangement is shown in FIG. 32.

In the FIG. 32 arrangement 134, the stacked panels are split into twosub-stacks to permit illumination by two different light sources. Thefirst light source 136 is a tungsten-halogen incandescent lamp, whichproduces a spectrum that is strong in red, especially when the lamp'soperating voltage is decreased, which may be desired to increase thelamp's life. The second light source 138 is a mercury arc-lamp, whichproduces a spectrum rich in deep blue light (430 nm), with a largeamount of energy also in the mid-green (540 nm) portion of the spectrum.The complementary spectrums produced by these two light sources areadvantageously combined in the embodiment of FIG. 32 to achieve goodbrightness, long lamp life and high color temperature "white."

As shown in FIG. 32, light from the tungsten-halogen lamp 136 follows afirst optical path that includes a holographic or dichroic mirror 139.(This mirror may be designed to pass all of the spectrum except a narrownotch [20 or 30 nm] at 540 nm.) This filtered light continues on toilluminate a stacked assembly 140 that includes red- andgreen-controlling panels (i.e. "cyan" and "magenta"). (For clarity ofpresentation, the polarizers, collimator, and other optical elementsused in this stack and elsewhere in the FIG. 32 embodiment are notillustrated. Following the logic discussed above, the entrance polarizeron the magenta panel may be red, the exit polarizer on the cyan panelmay be green, and the intermediate polarizer may be neutral.) The lightexiting the stacked assembly 140 is reflected off mirrors 142 and 144and is directed to exit optics for projection or direct viewing. Thetungsten-halogen light 136 thus provides illumination at the red andgreen portions of the spectrum, and the stacked assembly 140 controlsthese colors.

Light from the second, mercury arc-lamp 138 follows a second opticalpath towards a dichroic mirror 146, which reflects green light up tomirror 139 (for additional illumination of the red/green control stack140) and passes blue light into a blue controlling (i.e. "yellow") LCD148. The light exiting this blue LCD continues to and through the mirror144, joining with the red and green light and continuing to the exitoptics. A full color image is thus produced.

(It will be recognized that splitting the light in this fashion providessome design freedom, viz., that the red- and green-controlling panelscan be tuned without regard to their blue performance [since theyencounter no blue light] and the blue-controlling panel can be similarlytuned without regard to its red and green performance.)

If increased contrast is desired, a black/white panel may be included ineither the first or second optical paths. Alternatively, an additionalmagenta (i.e. green controlling) cell may be included in the stack 140since green is the dominant contributor to photopic brightness.

While the FIG. 32 embodiment provides different optical paths fordifferent portions of the optical spectrum, in other embodiments thedifferent optical paths can be dedicated to different polarizations oflight. Such split-by-polarization systems offer improved brightnesssince the cross-polarized light that is filtered from single pathsystems is instead directed to a second path where it is utilized.

FIG. 33 shows a system 150 similar to that of FIG. 32, except the FIG.33 system uses a single light source 152. Blue light from this lightsource is stripped off by a dichroic mirror 154, reflected off a mirror156, collimated by a collimator 158, and directed into a bluecontrolling LCD assembly 160. Light exiting this LCD assembly is focusedby a lens 162 through a blue-passing mirror 164 and into a lens 166 forprojection onto a viewing screen.

The red/green light from lamp 152 passes through mirror 154, iscollimated by a collimator 168, and illuminates a stack 170 thatincludes cyan and magenta panels (which control red and green light,respectively). The light exiting the stack 170 is again focused by alens 172, reflected off the mirror 164 and directed into the projectionlens 166.

Alternative Embodiments

While the foregoing description has focused on a single class ofembodiments that incorporate supertwisted nematic panels, it will berecognized that a variety of other embodiments can be constructed usingdifferent display elements. Exemplary is the display 300 shown in FIG.35. In this display, a thin film transistor (TFT) liquid crystal panel302 is operated in conjunction with an optical element 304 that exhibitsan electrically controllable birefringent (ECB) effect. Thin filmstransistors are known in the art and disclosed, inter alia, in U.S. Pat.Nos. 4,821,092, 4,819,038, 4,816,885, 4,776,673, 4,743,099, 4,743,098,4,723,838, 4,715,930, 4,654,117, 4,649,383, 4,636,038, 4,621,260,4,599,246, 4,591,848, 4,581,619, 4,461,071, 4,386,352, 4,385,292,4,299,447 and 3,824,003, the disclosures of which are incorporatedherein by reference.

In the illustrated embodiment, a limited birefringence mode liquidcrystal effect may be given to the TFT panel by adding a retardationfilm to the (90°) TN of a standard TFT-LCD and by selecting thepolarization orientation appropriately.

In the preferred embodiment, the TFT is optimized for whitest white(instead of blackest black). The TFT is also tuned to broaden the dip inthe spectral transmissivity curve and place it at the appropriatewavelength required by the stacked combination of panels. In some suchembodiments, a double dip in this curve may be obtained by use of aretardation film. By providing several layers of retardation film, ideal"notch filter" performance may more nearly be achieved.

The ECB element may be a supertwisted nematic panel detailed earlier, ormay comprise a conventional twisted nematic cell, or a great variety ofother elements, such as electro-optic or electro-acoustic crystals. (STNcells have generally not been used in a classical electricallycontrolled birefringence mode due to the very restricted range ofoperating voltages dictated by multiplexibility requirements. Rather,they have been operated in a bistable mode, operating in either theselect or non-select states, not in between. The present inventionexploits the voltage-dependent birefringence exhibited by STN cellswithin the narrow R.M.S. operating range between V_(select) andV_(non-select) to achieve a broad range of intermediate birefringentcolors.)

The TFT panel 302 and ECB panel 304 are sandwiched between threepolarizers 306, 308 and 310. The subtractive coloration provided by eachpixel in the ECB display element 304 is a function of the signal drivingthat pixel. The range of colors produced by this variable birefringenceis augmented by one or more additional colors attainable by use of theTFT panel 302 to produce a full color display.

A related embodiment is shown in FIG. 36. In this figure, a displaysubassembly 320 comprises a white/yellow mode STN (or DSTN) panel 322, athin film transistor panel 324 and a color shutter assembly 326 instacked arrangement with associated polarizers. Color shutter assembliesare known in the art and are described, inter alia, in U.S. Pat. Nos.4,758,818, 4,726,663, 4,652,087, 4,635,051, 4,611,889 and 4,582,396, thedisclosures of which are incorporated herein by reference.

In the illustrated display subassembly 320, the color shutter isoperated in alternate frames to block red and green light, respectively(thus giving the appearance of cyan and magenta). The TFT provides avery fast switching speed. The STN cell 322 is relatively slower thanthe TFT, but the blue light it controls is relatively less perceptibleto the human eye, so the slower response speed is of littlesignificance.

The FIG. 36 embodiment is intended to lower the costs associated withmaking a high-information content full color LCD based display system.This embodiment requires only one (monochrome) TFT panel, as opposed tothree that may otherwise be used to control red, green and blue.

The STN 322 may be grey-scaled, using a single bit plane of RAM, to 8 or16 levels. It can provide sufficiently fast response time for movingimages.

The TFT 324 is operated at twice the normal frame rate (i.e. greaterthan or equal to 120 Hz) and, along with the color shutter 326, controlsthe red and green image fields sequentially. Both the color shutter andthe TFT (with their associated polarizers) are tuned to leak blue allthe time, thereby improving the color balance, especially when atungsten-halogen lamp is used.

In still another embodiment of the present invention, two LCD panels maybe stacked and operated independently to produce a full gamut of colors.A display assembly 330 according to this construction is shown in FIG.37. For expository convenience, the panels will be referenced as STNpanels 332, 334, although again, a variety of other technologies can beused.

STN panels 332 and 334 are fabricated with a higher value of Δnd thanthose STN panels illustrated earlier. An exemplary panel may have a 240degree twist angle, with a Δnd value of 1.4. The larger Δnd valueproduces a wider variation in the voltage variable coloration effect.When a high information content, multiplex-addressed LCD panel isgrey-scaled (either by PWM or multiple frame averaging), intermediatevoltages (between V_(select) and V_(non-select)) can be attained on eachpixel, despite the nearly-bistable switching behavior that characterizesSTN cells.

Between neutral polarizers, the first panel 332 can obtain, for example,the color gamut shown in FIG. 38. By using a blue (ideal characteristic,pure blue leakage only) polarizer, the gamut is as illustrated in FIG.39, instead.

As can be seen, the color gamut with the blue polarizer is shiftedtowards the blue, with the result that "yellow" cannot be obtained, but"white" is obtained instead. The "blue" may not be as pure as desiredideally, but human-factors experts suggest that a desaturated blue isbetter for communicating visual information.

If the first polarizer 336 is dyed blue, producing the color gamut shownin FIG. 39, the second panel 334 needs only to be able to make yellowand white to make the full gamut of saturated primary and secondarycolors. To be safe and insure a true black state, the leakages of thefirst and third polarizers 336, 338 must not overlap, so rather thanchoose a yellow polarizer for polarizer 338, it is somewhat preferableto choose an orange polarizer instead (FIG. 40). Of course, by tuningthe Δnd of the LCD and, optionally, adding additional layers ofbirefringent material (such as retardation films), various birefringencecolors can be made within each layer. A desirable display according tothis embodiment has the following characteristics:

1) good white and black states (overall contrast ≧10:1);

2) good color saturation (especially redness of red); and

3) extra good blue light transmission to counteract tungsten illuminant(to raise the color temperature, i.e. color correction).

By using various commercially available color polarizers, the displaysubassembly can be optimized for various purposes and with differentperformances for different needs.

Other possibly complimentary pairs of colors for polarizers 336 and 338are cyan and red, green and red, green and violet, green and magenta,and perhaps green and blue.

The magenta formed by electrically controlled birefringence is generallypoor, because the red edge is too soft and must be supplemented. Thus, ared polarizer is desirable, which provides excellent sharpness (i.e.steepness of edge between yellowish-green and red). The gamut of thefirst LCD 332 may thus be, inter alia, one of the following:

a) red/magenta/white;

b) red/yellow/white; or

c) magenta/yellow/white.

In these cases, the second panel 334 should subtract red, at least, socyan is its chief color and it may attain one of the following:

d) cyan/magenta/white;

e) green/cyan/white;

f) cyan/green/yellow/white; or

g) blue/cyan/white.

Preferred combinations include a+f, b+g or c+d, above. (a+e requiresalternating red and green to make yellow; b+e is undesirable because itrequires alternating red plus blue pixels to make magenta and cannotmake blue; a+d cannot make green, b+d cannot make blue, etc.)

While the display subassembly 330 has been described as operating in acolor subtractive mode, it will be recognized that the display may alsobe operated in an additive mode, either with adjacent pixels beingoperated together to add spatially, or with a single pixel in thestacked assembly being operated with alternate colors to add temporally.

If both liquid crystal panels 332, 334 are tuned to produce fourdistinct shades of color, they can be operated co-jointly to productsixteen colors without the need for grey scaling.

Of the foregoing combinations, the presently preferred is to tune panel332 to produce a color gamut extending from magenta, through yellow towhite, and dye polarizer 336 red. Panel 334 can be tuned to produce acolor gamut extending from cyan, through green and yellow to white, andpolarizer 338 can be dyed green. Again, partial compensation can be usedto optimize the various colors produced.

Concluding Remarks

It will be recognized that the use of colored polarizers withbirefringent cells provides a great deal of design freedom. The reasonis that a colored polarizer renders the response of a cell at certainwavelengths irrelevant. Taking as an example a magenta (green blocking)cell, the cell must normally be optimized for a number of factors: hightransmissivity of blue light in both the select and deselect states, lowtransmissivity of green light in the select state but hightransmissivity of green light in the deselect state, and hightransmissivity of red light in both states. Such optimization typicallyrequires a compromise of all factors. When used in conjunction with amagenta polarizer, however, the cell's transmissivity at blue and redbecomes only a minor concern. The magenta-dyed polarizer causes the cellto pass blue and red regardless of the characteristics of the cell. Withthe importance of these factors minimized, the design of the cell canfocus on just one factor--high contrast between the select and deselectstates for green--and no compromises need be made.

In optimizing the response of the cells, it is sometimes desirable touse additional optical components. For example, in order to tune therange of birefringence effects it is sometimes desirable to includeretardation films adjacent the cells.

A birefringent cell, operated in conjunction with a neutral polarizer,exhibits a sinusoidal transmissivity versus wavelength curve, as wasshown in FIG. 2. Taking again the example of a magenta (green blocking)cell, the cell's Δnd is selected so the minimum of its sinusoidal curvefalls somewhere in the green portion of the spectrum. This minimum,however, may be relatively narrow, permitting relatively large amountsof higher and lower wavelength green light to pass through thecell/polarizer combination. To broaden this "notch" in thetransmissivity curve, a retardation film may be employed. Whileretardation films are generally used to tune the cell's characteristics(i.e. to move the dip in the curve up or down in wavelength), the film'saction in reversing part of the cell's twist also serves to broaden thedip somewhat. Thus, the transmissivity curve of the magenta cell in itsdeselect state may be made to more nearly approximate the ideal (i.e. arectangular notch that encompasses all of green--500 to 600 nm.).

While the display subassembly has been described as including singlesupertwisted liquid crystal panels, other types of birefringent optics,such as double supertwisted panels or single panels embodying othertechnologies (such as electro-optic [i.e. lithium tantalum niobate],acoustic-optic, or PET cells) can alternatively be used. A higherresolution display can be achieved by stacking two or more cells foreach color, with the active lines on one cell overlapping active lineson the other, similar to the basic technique shown in U.S. Pat. No.4,448,490, the disclosure of which is incorporated by reference. Fasterswitching times can be achieved by stacking several thin panels for eachcolor, as disclosed in U.S. Pat. No. 4,547,043, the disclosure of whichis incorporated by reference. The basic principles of the invention arealso applicable to other display technologies, such as interferencecolor systems.

In other embodiments, certain birefringent panels may be stacked withoutintervening polarizers. For example, two panels (with or withoutdifferent Δnd) may be stacked without an intermediate polarizer toproduce white, yellow, green and cyan in the four combinations of selectstates. A green polarizer can be used on the outside layer, since greenis common to all these colors. Such an embodiment is especially valuablefor a white, magenta, cyan and blue combination, since overheadprojection needs more blue throughput, which may be obtained by use of a"pure blue" polarizer.

In still other embodiments, if any of the LCD birefringence colors arenot ideal, some attenuation of specific light wavelengths might enhancethe color gamut and overall contrast. For example, two polarizers mightbe used together, or a weak color filter compensator (i.e. aconventional gelatin filter) might be added.

It will be recognized that known grey scaling techniques can readily beapplied to the present invention to provide the full gamut of possiblecolors. In one such embodiment, grey scaling is applied to each of thethree colored cells. In another embodiment, grey scaling is appliedsimply to a fourth (typically black) cell included in the stack. U.S.Pat. Nos. 4,840,462, 4,840,460, 4,818,078, 4,766,430, 4,743,096,4,709,995, 4,560,982, 4,508,427, 4,427,978 and 4,043,640 teach variousgrey scaling techniques and are incorporated herein by reference.

It will further be recognized that the invention can be practiced withmore or less panels than the three or four illustrated. For example, itis advantageous to provide laboratory instruments, such as oscilloscopesand analyzers, with displays formed by stacking two supertwistedbirefringent panels with one or more colored polarizers. The display maytake virtually any of the forms discussed above and still be suitablefor inclusion in the instrument. While color gamut is compromisedsomewhat by such a two panel stack, brightness is increased and cost isreduced.

Finally, it will be recognized that many advantageous results can beachieved by cojointly operating several TFT panels, either in stacked orsplit-optic arrangement, rather than including a single TFT panel in astack with other panels, as particularly discussed above. Some suchembodiments, including certain of the split optic path embodiments, donot rely on exploitation of the birefringence effect. A number of suchembodiments may be realized by substituting TFT panels for the STNpanels in the illustrated embodiments. (In such embodiments, the neutralpolarizers typically provided on a TFT panel by the manufacturer may beremoved, any spectral deficiencies of the panel may be compensated forby retardation film [i.e. commercially available panels are particularlydeficient in the blue portion of the spectrum, which deficiency can bealleviated by retardation film], and colored polarizers may be added toachieve the benefits discussed earlier.)

In view of the wide variety of embodiments to which the principles ofour invention may be applied, it should be recognized that theillustrated embodiments are to be considered illustrative only and notas limiting the scope of the invention. Instead, we claim as ourinvention all such modifications as may come within the scope and spiritof the following claims and equivalents thereto.

We claim:
 1. A method of producing a color display comprising:providingfirst and second optical assemblies, each assembly including a pluralityof independently operable pixels, each of said pixels exhibiting abirefringent effect that varies in response to a signal applied thereto;stacking said assemblies with a pixel in the first assembly linearlyaligned with a corresponding pixel in the second assembly; interposing afirst polarizer between the assemblies and sandwiching the stack betweena second and a third polarizer wherein said second and third polarizersare colored complementary colors; and applying a first dynamic signal tothe pixels in the first assembly and applying a second dynamic signal tothe pixels in the second assembly so as to selectively change thebirefringent effect.
 2. The method of claim 1 which includes providing asupertwisted nematic liquid crystal panel in each of said first andsecond optical assemblies.
 3. The method of claim 1 which furtherincludes enhancing display brightness by interposing a polarizer dyed acolor other than black between the assemblies.
 4. The method of claim 1further comprising:applying said first dynamic signal to said pixels inthe first assembly wherein said first dynamic signal changes selectivelyand periodically between more than two values so that said seconddynamic signal differs from said first signal thereby producing morethan two distinct birefringent effects.
 5. The method of claim 1 whereineach assembly has a value of Δnd greater than 0.05 μm and isindividually tuned to produce a different plurality of colors incooperation with additional layers of birefringent material and saidpolarizers to produce a wider voltage variable coloration effect.
 6. Adisplay subassembly comprising:first, second and third supertwistednematic birefringent liquid crystal display panels; first, second, thirdand fourth polarizers; the first panel being positioned between thefirst and second polarizers, the second panel being positioned betweenthe second and third polarizers and the third panel being positionedbetween the third and fourth polarizers; the birefringence of the first,second and third panels, together with the polarizers adjacent thereto,cooperating to pass light of first, second and third colors,respectively, when the panels are in first states, and to passsubstantially all colors of light when the panels are in second states;each of said panels having a plurality of independently electronicallyoperable pixels, said panels being stacked so that corresponding pixelsin each of said panels are aligned; and at least said first or saidfourth polarizers is a color other than black so as to increase displaybrightness.
 7. The display subassembly of claim 6 wherein said secondpolarizer colored a color common to said first and second colors oflight passed by said first and second panels in the first state.
 8. Thedisplay subassembly of claim 6 wherein said third polarizer is colored acolor common to said second and third colors of light passed by saidsecond and third panels in the first state.
 9. The display subassemblyof claim 6 in which the first, second and third colors are eachsubtractive primary colors.
 10. A color display that can be operated toyield at least the colors black, white, red, green, blue, cyan, yellowand magenta comprising: an entrance and exit polarizer, each polarizerdyed a color other than black and at least a first and secondbirefringent subassembly disposed between said polarizers in a stackedrelationship, each of said subassemblies defining a plurality of pixels,each of said pixels being independently operable to birefringentlycontrol passage of a portion of a spectrum of light passingtherethrough, and wherein at least two of said subassemblies eachincludes a supertwisted nematic panel.
 11. The display of claim 10wherein said supertwisted nematic panels have substantially equal valuesof Δnd and a third neutral polarizer disposed between said firs andsecond subassemblies.
 12. The display of claim 11 in which the first andsecond subassemblies are stacked without an intervening polarizer. 13.The display of claim 11 in which the first and second subassemblies arestacked with an intervening polarizer therebetween.
 14. The display ofclaim 13 in which the intervening polarizer is dyed a color other thanblack.
 15. The display of claim 10 in which a supertwisted nematic panelin a first of the subassemblies has a different value of 66 nd than asupertwisted panel in a second of the subassemblies.
 16. The display ofclaim 15 in which the first and second subassemblies are stacked withoutan intervening polarizer.
 17. The display of claim 15 in which the firstand second subassemblies are stacked with an intervening polarizertherebetween.
 18. The display of claim 17 in which said interveningpolarizer is dyed a color other than black.
 19. The display of claim 10further comprising a third subassembly wherein, at least two of saidthree subassemblies include supertwisted nematic panels each with asubstantially equal value of Δnd..
 20. The display of claim 10 furthercomprising a third subassembly wherein, each of said subassembliesincludes a supertwisted nematic panel and each of said panels has asubstantially equal value of Δnd.
 21. The display of claim 20 in whichat least two of said subassemblies comprise a birefringent LCD panel andat least a passive compensation layer.
 22. The display of claim 21further comprising means for grey scaling said display to yield morethan eight colors.
 23. The display of claim 10 in which at least one ofthe subassemblies includes a passive compensation layer.
 24. The displayof claim 10 further comprising means for grey scaling said display toyield more than eight colors.
 25. A color display method comprising thesteps:providing first and second display subassemblies, each of saidassemblies including a supertwisted nematic birefringent panel, each ofsaid panels defining a plurality of independently operable pixels, atleast one of said display subassemblies additionally including a passivecompensation layer to tune the color response thereof; stacking saidsubassemblies with a first polarizer therebetween, said first polarizerbeing dyed a color other than a color intended to be blocked by saidfirst subassembly; sandwiching the stacked subassemblies between secondand third polarizers; operating pixels in each of the panelsindependently to birefringently control passage of a portion of aspectrum of light passing therethrough in cooperation with said first,second and third polarizers, wherein at least the eight colors black,white, red, green, blue, cyan, yellow and magenta may thereby bedisplayed; and controllably grey scaling at least one of thesupertwisted nematic panels to yield colors in addition to the aforesaideight colors.
 26. The method of claim 23 in which:the providing stepincludes providing a third display subassembly, said third subassemblyincluding a supertwisted nematic birefringent panel and a fourthpolarizer; the stacking step includes stacking said third subassemblywith the first and second subassemblies whereby said third displaysubassembly is disposed between said third and fourth polarizers; theoperating step including operating pixels in the third panel tobirefringently control passage of a portion of a spectrum of lightpassing therethrough in cooperation with said third and fourthpolarizers; and the grey scaling step including grey scaling each of thethree supertwisted nematic panels.
 27. The method of claim 26 in whichthe providing step includes providing first, second and third displaysubassemblies, each of said subassemblies including supertwisted nematicbirefringent panels with the same values of Δnd, and at least two ofsaid subassemblies each including a passive compensation layer.
 28. Anaccessory for use with a conventional overhead projector to projectelectronically generated color images therefrom, the projector having abulb, a projection lens, and a projection surface having a Fresnel lensthereunder, the Fresnel lens focusing light from the bulb into theprojection lens, the accessory being adapted to rest on the projectionsurface and comprising:first lens means for receiving converging whitelight from the projection surface of the projector and collimating saidlight; second lens means for receiving collimated light and focusingsaid light into the projection lens; a display subassembly positionedbetween the first and second lens means and illuminated by thecollimated light, said subassembly including first, second, third andfourth polarizers and further including first, second and thirdtransmissive liquid crystal birefringent display panels, thebirefringence of said panels being tuned, respectively, to first, secondand third subtractive complementary colors, each of said panels having aplurality of electronically operable pixels, said panels being stackedso that corresponding pixels in each of said panels are aligned with thedirection of the collimated light, the first panel being positionedbetween the first and second polarizers, the second panel beingpositioned between the second and third polarizers and the third panelbeing positioned between the third and fourth polarizers.
 29. Theaccessory of claim 28 in which the first polarizer is colored the firstcolor.
 30. The accessory of claim 29 in which the fourth polarizer iscolored the third color.
 31. The accessory of claim 28 in which thesecond polarizer is colored a color other than the first or second coloror black.
 32. The accessory of claim 28 in which:the first polarizer iscolored the first color; the fourth polarizer is colored the thirdcolor; the second polarizer is colored a color passed by both the firstand second panels; and the third polarizer is colored a color passed byboth the second and third panels.
 33. The accessory of claim 28 inwhich:one of the panels is yellow; one of the panels is cyan; one of thepanels is magenta; the colors of the first and fourth polarizers areselected from the list: yellow, cyan, magenta and black; and the colorsof the second and third polarizers are selected from the list: red,green, blue and black; wherein each of the polarizers is coloreddifferently.
 34. A method of projecting a colored image, comprising thesteps:providing first, second and third birefringent liquid crystalpanels that include a plurality of independently selectable pixels, saidpanels being tuned to selectively pass first, second and thirdsubtractive color primaries, respectively, when a signal is applied toeach panel; stacking said first, second and third panels so thatcorresponding pixels in each of said panels are aligned along an axisorthogonal thereto, said stacked panels forming a subassembly; providinga first polarizer between the first and second panels; providing asecond polarizer between the second and third panels; and sandwichingsaid subassembly and said first and second polarizers between a thirdpolarizer, colored the first subtractive primary color, and a fourthpolarizer, colored the third subtractive primary color; passing lightthrough the subassembly in a direction parallel to the axis of alignmentand selectively-controlling spectral portions of said light when asignal is applied to pixels on at least one of said panels; focusing thelight exiting the subassembly parallel to the axis of alignment into aprojection lens; and projecting the light exiting the projection lensonto a display screen.
 35. The method of claim 34 which further includesthe step of collimating the light parallel to the axis of alignment ofthe panels prior to passage into the subassembly.
 36. A color displaycomprising:first and second optics; first, second and third displaysubassemblies disposed between said optics; each of said displaysubassemblies including a liquid crystal panel defining a plurality ofindependently operable pixels wherein at least two of said three liquidcrystal panels control color through use of a birefringence effect andwherein at least two of said display subassemblies have different valuesof Δnd; first, second, third and fourth polarizers, at least one of saidfour polarizers dyed a color other than black; said first displaysubassembly disposed between the first and second polarizers; saidsecond display subassembly disposed between the second and thirdpolarizers; and said third display subassembly disposed between thethird and fourth polarizers.
 37. The color display of claim 36 in whichthe liquid crystal panels each comprises supertwisted nematic liquidcrystal panels.
 38. The color display of claim 36 in which at least oneof the display subassemblies includes:a birefringent liquid crystalpanel; a first passive layer that minimizes the birefringent effectexhibited by the birefringent liquid crystal panel; and a second passivelayer that introduces a birefringent effect.
 39. The color display ofclaim 36 in which at least one of said four polarizers is colored acolor other than black.
 40. The color display of claim 36 in which oneof said display subassemblies operates, in conjunction with thepolarizers adjacent thereto, to pass all colors of light in a firststate and to pass all colors of light except blue in a second state. 41.The color display of claim 36 in which:at least one of said fourpolarizers is colored a color other than black; one of said displaysubassemblies operates, in conjunction with the polarizers adjacentthereto, to pass all colors of light in a first state and to pass allcolors of light except blue in a second state.
 42. The color displaysystem of claim 41 in which said first optic comprises a Fresnel lensadapted to collimate converging light introduced thereto, and the secondoptic comprises a Fresnel lens adapted to cause said collimated lightexiting therefrom to substantially converge.
 43. A color displayaccording to claim 42 that is operable to produce at least the colorsblack, white, red, green, blue, cyan, yellow and magenta.
 44. The colordisplay of claim 36 in which the first optic includes a mirror, and thesecond optic includes a translucent screen having a back surface onwhich an image can be displayed and a front surface from which the imagecan be viewed, said screen being disposed about a spring loaded rollerfrom which it can be unrolled for use and rolled for storage.
 45. Amethod of producing a color display comprising the steps:collimatingilluminating light; linearly polarizing said collimated light with afirst polarizer; passing said collimated polarized light through a firstdisplay subassembly that includes a liquid crystal panel with aplurality of independently operable pixels; said passing step includingelliptically polarizing the linearly polarized light so that differentwavelengths of light are rotated to different angular orientations;analyzing light exiting the first display subassembly with a secondpolarizer to attenuate therefrom wavelengths of light cross polarizedrelative to the second polarizer, said analyzing step thereby yieldinglinearly re-polarized light from which certain wavelengths have beenattenuated; passing said linearly re-polarized light through a seconddisplay subassembly that includes a liquid crystal panel with aplurality of independently operable pixels; said second passing stepincluding elliptically polarizing the linearly re-polarized light sothat different wavelengths of light are rotated to different angularorientations; analyzing light exiting the second display subassemblywith a third polarizer to attenuate therefrom wavelengths of light crosspolarized relative to the third polarizer, this analyzing step therebyyielding linearly polarized light from which additional wavelengths havebeen attenuated; and converging the linearly polarized light from thethird polarizer, wherein: the elliptical polarization effected duringthe first passing step differs from elliptical polarization effectedduring the second passing step due to different values of Δnd exhibitedby the first and second display subassemblies; and at least one of saidfirst, second or third polarizers is dyed a color other than black; andthe method further includes: applying a first dynamic signal to a pixelin the first liquid crystal panel; and applying a second dynamic signaldifferent from the first dynamic signal to a pixel in the second liquidcrystal panel.
 46. The method of claim 45 which further includesindependently varying the first and second dynamic signals among morethan two values.
 47. The method of claim 45 in which the passing stepseach further includes passing light through a supertwisted nematicliquid crystal panel.
 48. The method of claim 45 in which at least oneof the passing steps includes passing light through:a birefringentliquid crystal panel; a first passive layer that minimizes thebirefringent effect exhibited by the birefringent liquid crystal panel;and a second passive layer that introduces a birefringent effect.
 49. Acolor display comprising, in combination: first and second displaysubassemblies each having a net Δnd value greater than 0.5 μm, eachincluding a liquid crystal panel defining a plurality of independentlyoperable pixels, which exhibit a birefringent effect that varies inresponse to a signal applied thereto, said first subassembly having anet Δnd value at least 0.05 μm greater than the net Δnd value of thesecond subassembly;a first polarizer interposed between the two displaysubassemblies; and a second and a third polarizer sandwiched about thetwo display subassemblies wherein at least one of the first, second orthird polarizers is colored a color other than black.
 50. The colordisplay of claim 49 in which the first and second liquid crystal panelsare each supertwisted nematic liquid crystal panels.
 51. The colordisplay of claim 49 in which at least one of the first and second liquidcrystal panels comprise:a birefringent liquid crystal panel; a firstpassive layer that minimizes the birefringent effect exhibited by thebirefringent liquid crystal panel; and a second passive layer thatintroduces a birefringent effect.
 52. The color display of claim 49which further comprises:a third display subassembly having a net Δndvalue at least 0.05 μm greater than the net Δnd values of the first andsecond panels; said third display subassembly including a liquid crystalpanel and defining a plurality of independently operable pixels; afourth polarizer; and said third display subassembly interposed betweenthe third and fourth polarizers; wherein: all three of the subassemblieshave Δnd values greater than 0.5 μm.
 53. A subtractive color displaymethod that includes the steps:providing a birefringent displaysubassembly that includes a nematic liquid crystal cell and that definesa plurality of independently operable pixels, wherein a birefringenteffect exhibited by one of said pixels varies in response to anelectrical signal applied thereto; operating the display subassembly inconjunction with entrance and exit polarizers; linearly polarizing lightentering the pixel by passing it through the entrance polarizer;elliptically polarizing said linearly polarized light as it passesthrough the pixel, whereby different wavelengths of light are orientedat different angular orientations when they exit the pixel, theelliptical polarization when a first electrical signal is applied tosaid pixel differing from the elliptical polarization when a secondelectrical signal is applied to said pixel; analyzing the ellipticallypolarized light exiting the pixel so that light oriented orthogonally toan axis of the exit polarizer is attenuated by the exit polarizer;wherein:when the first electrical signal is applied to the pixel, thebirefringence of the pixel orients a first wavelength of lightorthogonally to the axis of the exit polarizer, thereby causing itsattenuation by the exit polarizer; and when the second electrical signalis applied to the pixel, the different birefringence of the pixelorients the first wavelength of light so that it is no longer orientedorthogonally to the axis of the exit polarizer, thereby permitting it topass through the exit polarizer relatively less attenuated; wherein theexit polarizer is dyed to pass wavelengths of light, other than thefirst wavelength, relatively unattenuated regardless of theirorientation, thereby improving the transmission of said wavelengths oflight and optimizing the resulting display brightness of the subtractivecolor display.
 54. A subtractive color display method that includes thesteps:providing a birefringent display subassembly that includes anematic liquid crystal cell and that defines a plurality ofindependently operable pixels, wherein a birefringent effect exhibitedby one of said pixels varies in response to an electrical signal appliedthereto; operating the display subassembly in conjunction with entranceand exit polarizers; linearly polarizing light of a first wavelengthentering the pixel by passing it through the entrance polarizer;elliptically polarizing light as it passes through the pixel, wherebydifferent wavelengths of light are oriented at different angularorientations when they exit the pixel, the elliptical polarization whena first electrical signal is applied to said pixel differing from theelliptical polarization when a second electrical signal is applied tosaid pixel; analyzing the elliptically polarized light exiting the pixelso that light oriented orthogonally to an axis of the exit polarizer isattenuated by the exit polarizer, wherein:when the first electricalsignal is applied to the pixel, the birefringence of the pixel orientsthe first wavelength of light orthogonally to the axis of the exitpolarizer, thereby causing its attenuation by the exit polarizer; andwhen the second electrical signal is applied to the pixel, the differentbirefringence of the pixel orients the first wavelength of light so thatit is no longer oriented orthogonally to the axis of the exit polarizer,thereby permitting it to pass through the exit polarizer relatively lessattenuated; wherein the entrance polarizer is dyed to pass wavelengthsof light, other than the first wavelength, relatively unattenuatedregardless of their orientation, thereby improving the transmission ofsaid wavelengths of light and optimizing the resulting displaybrightness of the subtractive color display.
 55. In a color displaysystem comprising:a first, second and third polarizer; first and secondcells comprising supertwisted nematic type liquid crystal material, saidcells exhibiting a birefringent effect and having different values ofΔnd, said cells having a plurality of independently operable pixels anddisposed in a stacked arrangement with said polarizers; said firstpolarizer interposed between said first and second cells and said secondand third polarizers disposed outside the stacked arrangement adjacentto said first and second cells, respectively, whereby said first cell incooperation with said first and second polarizers provides spectralresponse different from said second cell in cooperation with said firstand third polarizers.
 56. The color display system of claim 55 furthercomprising a passive compensation layer adjacent to one of said cells.57. The color display system of claim 55 which further includes at leasttwo polarizers adjacent different surfaces of the first and secondportions, each of said two polarizers being dyed a color other thanblack.
 58. A color display system comprising:a first and second portionin stacked arrangement; each portion including an optical elementdefining a plurality of independently operable pixels and exhibiting abirefringent effect that varies in response to an applied signal; afirst polarizer interposed between said first and second portion; asecond and third polarizer sandwiched about said stacked portions, saidsecond and third polarizers colored a color other than black so that theleakage characteristics of said second and third polarizers do notoverlap; the first portion, in combination with said first and secondpolarizers, exhibits a first spectral response when driven with a firstsignal; and the second portion, in combination with said first and thirdpolarizers, exhibits a second spectral response when driven with thefirst signal.
 59. The color display system of claim 58 in which theportions exhibit different spectral responses and different values ofΔnd, each of which is greater than 0.50 μm.
 60. The color display systemof claim 58 in which the different spectral responses are achieved byinclusion of a passive optical element in one of said portions.
 61. Thecolor display system of claim 60 in which the passive element is aretardation film.
 62. A color display comprising:first and secondstacked birefringent display subassemblies, each having a supertwistednematic liquid crystal panel defining a plurality of independentlyoperable pixels, said first and second display subassemblies stacked sothat corresponding pixels in each of said subassemblies are linearlyaligned, at least one of said display subassemblies having a value ofΔnd greater than 1.05 micrometers; a first polarizer disposed betweensaid first and second subassemblies; second and third polarizerssandwiched about said first and second subassemblies, said second andthird polarizers colored different primary colors selected from thelist: red, green and blue whereby each pair of corresponding pixels inthe first and second display subassemblies can be cooperatively operatedto produce all the colors comprising red, green, blue, yellow, magenta,cyan, black and white.
 63. The color display of claim 62 wherein:thefirst and second display subassemblies each includes a supertwistednematic panel; at least one of said display subassemblies exhibits avalue of Δnd greater than 1.05 micrometers; and each pair ofcorresponding pixels in the first and second display subassemblies canbe cooperatively operated to produce all the primary and secondarycolors (red, green, blue, yellow, magenta and cyan), together with blackand white; wherein a true, eight color display can be produced with astack of just two liquid crystal panels.
 64. The color display of claim63 wherein:at least one of the display subassemblies exhibits a value ofΔnd greater than 1.15 micrometers. .Iadd.65. A color liquid crystaldisplay, comprising: first and second subtractive LCD filters, eachfilter, comprising means for independently subtracting one of theprimary colors (red, green, blue) from a polychromatic light beamwithout substantially affecting the other primary colors, each filterincluding: an entrance selective polarizer for polarizing light of oneof the primary colors while passing light of the other colorssubstantially unpolarized; a LCD panel for selectively changingpolarization of incident light; and an exit selective polarizer forselectively blocking or passing the light of the same one of the primarycolors, depending upon the polarization change imparted by the LCDpanel, and passing light of the other colors. .Iaddend..Iadd.66. Theliquid crystal display of claim 65 in which:the first subtractive LCDfilter comprises a yellow filter for subtracting blue light; and thesecond subtractive LCD filter comprises a magenta filter for subtractinggreen light. .Iaddend..Iadd.67. The liquid crystal display of claim 66in which the first-second polarizer comprises a red polarizer forpolarizing preen and blue light. .Iaddend..Iadd.68. The liquid crystaldisplay of claim 65 in which sequentially adjacent entrance and exitpolarizers of the first and second filters comprise a single polarizer..Iaddend..Iadd.69. The liquid crystal display of any of claims 65-67 inwhich each LCD panel comprises a plurality of individually selectableregions. .Iaddend..Iadd.70. The liquid crystal display of claim 65 inwhich the LCD panel employs a birefringence optical operating mode toselectively chance polarization of incident light. .Iaddend..Iadd.71.The liquid crystal display of claim 65 in which the LCD panel includes asupertwisted nematic LCD cell. .Iaddend..Iadd.72. Display apparatuscomprising:first and second liquid crystal display panels for impartingan effective polarization change to applied incident spectral radiantenergy as an excitation voltage is applied to the liquid crystal displaypanel; first, second, third and fourth polarizers; the first liquidcrystal display panel being disposed between the first and secondpolarizers, the second liquid crystal display panel being disposedbetween the third and fourth polarizers; the first and second polarizerscharacterized by selectively linearly polarizing the spectral radiantenergy of a first primary color, while transmitting the spectral energyof second and third primary colors unaffected; the third and fourthpolarizers selected to linearly polarize spectral radiant energy of thesecond primary color, while transmitting spectral radiant energy of thefirst and third primary colors unaffected; and the first and secondpanels in cooperation with the selective polarizers adjacent theretobeing operative, in combination, to block one of the primary colors,while transmitting the balance of incident spectral radiant energyunaffected when the panel is in a first state and for transmittingincident spectral radiant energy of all colors when the panel is in asecond state. .Iaddend..Iadd.73. The display apparatus of claim 72 inwhich each of said panels comprises an array of electronicallycontrolled pixels, corresponding ones of said pixels in each of saidpanels being aligned along an axis or orthogonal to the panels..Iaddend..Iadd.74. The display apparatus of claim 72 in which thecombination of the first panel and the first and second selectivepolarizers adjacent thereto is operative to linearly polarize spectralenergy of a first primary color when the panel is in a second state,while transmitting the spectral energy for the second and third primarycolors unpolarized. .Iaddend..Iadd.75. The display apparatus of claim 72in which the combination of the second panel and the third and fourthselective polarizers adjacent thereto is operative to linearly polarizethe spectral energy of a second primary color when the panel is in asecond state, while transmitting the spectral energy for the first andthird primary colors unpolarized. .Iaddend..Iadd.76. The displayapparatus of claim 72 in which the first and second liquid crystaldisplay panels employ a birefringence optical operating mode to imparteffective polarization chances to incident spectral radiant energy..Iaddend..Iadd.77. The display apparatus of claim 72 in which the firstand second liquid crystal display panels each include a supertwistednematic LCD cell. .Iaddend..Iadd.78. Display apparatus comprising:firstand second liquid crystal display panels for imparting an effectivepolarization change to applied incident spectral radiant energy as anexcitation voltage is applied to the liquid crystal display panels;first, second and third polarizers; the first panel disposed between thefirst and second polarizers, the second panel disposed between thesecond and third polarizers; the first polarizer being adapted toselectively linearly polarize spectral radiant energy corresponding to afirst primary color, while transmitting spectral radiant energy of othercolors substantially unaffected; the second polarizer operative toselectively linearly polarize spectral radiant energy of both the firstand second primary colors, while transmitting spectral radiant energy ofother colors substantially unaffected; the third polarizer beingselected to selectively linearly polarize the second primary color whiletransmitting other colors unpolarized; and the first and second panelsin cooperation with the selective polarizers adjacent thereto beingoperative, in combination, to block one of the primary colors whiletransmitting the balance of incident spectral radiant energy unaffectedin a first state, and to pass incident spectral radiant energy of allcolors in a second state. .Iaddend..Iadd.79. The display apparatus ofclaim 78 in which each of said panels comprises an array ofelectronically controlled pixels, corresponding ones of said pixels ineach of said panels being aligned alone an axis or orthogonal to thepanels. .Iaddend..Iadd.80. The display apparatus of claim 78 in whichthe first liquid crystal display panel, in the second state, incombination with the first and second selective polarizers adjacentthereto, linearly polarizes the spectral energy of the first and secondprimary colors, while transmitting the spectral energy of the thirdprimary color unpolarized. .Iaddend..Iadd.81. The display apparatus ofclaim 78 in which the first and second liquid crystal display panelsemploy a birefringence optical operating mode to impart effectivepolarization changes to incident spectral radiant energy..Iaddend..Iadd.82. The display apparatus of claim 78 in which the firstand second liquid crystal display panels each include a supertwistednematic LCD cell. .Iaddend.